The present invention relates to a method for producing a magnetic recording medium having a magnetic layer formed of a metal thin film and used for a magnetic tape, magnetic disk, magnetic card, or the like. Particularly, the invention relates to a method for producing a magnetic recording medium excellent both in corrosion resistance and in durability.
For magnetic recording, heretofore so-called coating-type magnetic recording media have been used which are formed by applying magnetic powder onto a substrate with the magnetic powder particles being enveloped in an organic binder. On the other hand, in response to a recent demand for high-density recording media having improved performance characteristics, magnetic layers used in magnetic media have had to be made thinner while having a high coercive force and high saturation flux density. Magnetic recording media of the metal thin-film type having magnetic layers formed of metal thin films have been developed. Generally, the metal thin film does not contain appreciable amounts of non-magnetic components (compared with the aforementioned coating type magnetic film), and consequently the metal thin film has a high saturation flux density.
Methods for producing such magnetic metal thin films are classified into two types, one being a method carried out by means of a wet treatment, such as electrolytic plating, electroless plating, or the like, and the other being carried out by means of a dry treatment, such as vacuum evaporation, ion plating, sputtering, or the like. Particularly, the method of the latter type employing a dry treatment has been of interest from the viewpoints of manufacturing efficiency, reproducibility and stability, and various research on such methods has been carried out. For example, such research has included investigations of oblique-evaporated films (socalled evaporated tapes) of cobalt alloy formed by vacuum evaporation, perpendicular magnetic films of cobalt-chrome alloy formed by sputtering, and the like.
Although the recording media of the metal thin film type are ideal with respect to magnetic characteristics and electromagnetic conversion characteristics as described above, such recording media are inferior in corrosion resistance because of the innate properties of metal thin films. The corrosion resistance problem has dominated the research on magnetic recording media of the metal thin film type.
Various proposals have been made as to countermeasures. Examples of these proposals include oxidizing, nitrifying or carbonating the surface of the magnetic metal thin film and the like (as disclosed in Japanese Patent Unexamined Publication No. 50-33806), applying a protective film of SiO.sub.2, SiC or the like onto the metal thin film (as disclosed in Japanese Patent Unexamined Publication No. 50-104602), and adding additive elements excellent in corrosion resistance to the magnetic metal components (as disclosed in Japanese Patent Unexamined Publication No. 59-61013). However, all of these techniques have a disadvantage in that the magnetization properties of the resulting recording media are deteriorated. Therefore, a way of maintaining corrosion resistance without loss of the original advantages of metal thin film media has been sought.
The term "corrosion resistance" actually includes several separable concepts. That is, corrosion involves several different aspects, which include occurrence of rust due to so-called dew condensation or spontaneous dew generation (or cyclic dew generation), damage due to fine-grained salt particle (mainly in seaside regions), and damage due to corrosive gases such as H.sub.2 S gas, SO.sub.2 gas, NO.sub.x gas, or the like.
Heretofore, the estimation of the corrosion resistance of metal thin film type magnetic recording media has been carried out mainly based on the occurrence of rust as a result of dew condensation and on the decrease of saturation flux after maintaining the medium in an atmosphere of high temperature and high humidity (for example, 60.degree. C. and 90% RH). Also, the present applicants have carried out a corrosion test with respect to a corrosive gas upon various types of magnetic recording media, and consequently have found that metal thin film type magnetic recording media, especially metal thin film media containing cobalt as a main component, are inferior in corrosion resistance against SO.sub.2 gas. SO.sub.2 gas pollution is unfortunately present in many large cities, industrial areas, and other areas. Accordingly, magnetic recording media must have corrosion resistance against SO.sub.2 to make those media fit for practical use.